Ablation catheter having a virtual electrode comprising portholes and a porous conductor

ABSTRACT

An ablation catheter used for treatment of, for example, atrial fibrillation by electrically isolating a vessel, such as a pulmonary vein, from a chamber, such as the left atrium. The ablation catheter has a virtual electrode and a catheter shaft. The virtual electrode comprises a porous conductor. The catheter shaft includes a proximal portion and a distal portion. The distal portion includes an active region, which is either a looped structure transverse to the longitudinal axis of the catheter shaft, or a linear structure that extends parallel to the longitudinal axis of the catheter shaft. During use, the active region is directed into contact with, for example, the wall of a pulmonary vein and, upon energization, the virtual electrode creates a continuous lesion at or near the ostium of the pulmonary vein, thereby electrically isolating the pulmonary vein from the left atrium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/608,297, filed 27 Jun. 2003 (the '297 application), now U.S. Pat. No.6,960,207, which claims priority to U.S. provisional application No.60/441,849, filed 21 Jan. 2003 (the '849 application). The '297 and '849applications are hereby incorporated by reference as though fully setforth herein. This application is related to U.S. application Ser. No.10/347,034, filed 17 Jan. 2003 (the '034 application). The '034application is hereby incorporated by reference as though fully setforth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates to catheters for diagnosing and treating tissue,particularly human cardiac tissue. In particular, the invention relatesto an ablation catheter comprising a virtual electrode at a distalportion of the catheter to ablate tissue, the virtual electrode usingenergy emanating from a porous conductor contained within the distalportion.

b. Background Art

Catheters have been in use for medical procedures for many years.Catheters can be used for medical procedures to examine, diagnose, andtreat while positioned at a specific location within the body that isotherwise inaccessible without more invasive procedures. During theseprocedures a catheter is inserted into a vessel near the surface of thebody and is guided to a specific location within the body forexamination, diagnosis, and treatment. For example, one procedureutilizes a catheter to convey an electrical stimulus to a selectedlocation within the human body. Another procedure utilizes a catheterwith sensing electrodes to monitor various forms of electrical activityin the human body.

Catheters are also used increasingly for medical procedures involvingthe human heart. Typically, the catheter is inserted in an artery orvein in the leg, neck, or arm of the patient and threaded, sometimeswith the aid of a guide wire or introducer, through the vessels until adistal tip of the catheter reaches the desired location for the medicalprocedure in the heart.

A typical human heart includes a right ventricle, a right atrium, a leftventricle, and a left atrium. The right atrium is in fluid communicationwith the superior vena cava and the inferior vena cava. Theatrioventricular septum separates the right atrium from the rightventricle. The tricuspid valve contained within the atrioventricularseptum provides communication between the right atrium and the rightventricle.

In the normal heart, contraction and relaxation of the heart muscle(myocardium) takes place in an organized fashion as electro-chemicalsignals pass sequentially through the myocardium from the sinoatrial(SA) node, which comprises a bundle of unique cells disposed in the wallof the right atrium, to the atrioventricular (AV) node and then along awell-defined route, which includes the His-Purkinje system, into theleft and right ventricles. The AV node lies near the ostium of thecoronary sinus in the interatrial septum in the right atrium. Each cellmembrane of the SA node has a characteristic tendency to leak sodiumions gradually over time such that the cell membrane periodically breaksdown and allows an inflow of sodium ions, thereby causing the SA nodecells to depolarize. The SA node cells are in communication with thesurrounding atrial muscle cells such that the depolarization of the SAnode cells causes the adjacent atrial muscle cells to depolarize. Thisresults in atrial systole, wherein the atria contract to empty and fillblood into the ventricles. The atrial depolarization from the SA node isdetected by the AV node which, in turn, communicates the depolarizationimpulse into the ventricles via the bundle of His and Purkinje fibersfollowing a brief conduction delay. The His-Purkinje system begins atthe AV node and follows along the membranous interatrial septum towardthe tricuspid valve through the atrioventricular septum and into themembranous interventricular septum. At about the middle of theinterventricular septum, the His-Purkinje system splits into right andleft branches which straddle the summit of the muscular part of theinterventricular septum.

Sometimes abnormal rhythms occur in the heart, which are referred togenerally as arrhythmia. For example, a common arrhythmia isWolff-Parkinson-White syndrome (W-P-W). The cause of W-P-W is generallybelieved to be the existence of an anomalous conduction pathway orpathways that connect the atrial muscle tissue directly to theventricular muscle tissue, thus bypassing the normal His-Purkinjesystem. These pathways are usually located in the fibrous tissue thatconnects the atrium and the ventricle.

Other abnormal arrhythmias sometimes occur in the atria, which arereferred to as atrial arrhythmia. Three of the most common atrialarrhythmia are ectopic atrial tachycardia, atrial fibrillation, andatrial flutter. Atrial fibrillation can result in significant patientdiscomfort and even death because of a number of associated problems,including the following: an irregular heart rate, which causes patientdiscomfort and anxiety; loss of synchronous atrioventricularcontractions, which compromises cardiac hemodynamics, resulting invarying levels of congestive heart failure; and stasis of blood flow,which increases the likelihood of thromboembolism.

Efforts to alleviate these problems in the past have includedsignificant usage of pharmacological treatments. While pharmacologicaltreatments are sometimes effective, in some circumstances drug therapyhas had only limited effectiveness and is frequently plagued with sideeffects, such as dizziness, nausea, vision problems, and otherdifficulties.

An increasingly common medical procedure for the treatment of certaintypes of cardiac arrhythmia is catheter ablation. During conventionalcatheter ablation procedures, an energy source is placed in contact withcardiac tissue to heat the tissue and create a permanent scar or lesionthat is electrically inactive or noncontractile. During one procedure,the lesions are designed to interrupt existing conduction pathwayscommonly associated with arrhythmias within the heart. The particulararea for ablation depends on the type of underlying arrhythmia. Onecommon ablation procedure treats atrioventricular nodal reentranttachycardia (AVNRT). Ablation of fast or slow AV nodal pathways isdisclosed in Singer, I., et al., “Catheter Ablation for Arrhythmias,”Clinical Manual of Electrophysiology, pgs. 421-431 (1993). The use ofelectrode catheters for ablating specific locations within the heart hasalso been disclosed in, for example, U.S. Pat. Nos. 4,641,649,5,228,442, 5,231,995, 5,263,493, and 5,281,217.

Another medical procedure using ablation catheters with sheaths toablate accessory pathways associated with W-P-W utilizing both atransseptal and retrograde approach is discussed in Saul, J. P., et al.,“Catheter Ablation of Accessory Atrioventricular Pathways in YoungPatients: Use of long vascular sheaths, the transseptal approach and aretrograde left posterior parallel approach,” Journal of the AmericanCollege of Cardiology, Vol. 21, no. 3, pgs. 571-583 (1 Mar. 1993). Othercatheter ablation procedures are disclosed in Swartz, J. F.,“Radiofrequency Endocardial Catheter Ablation of AccessoryAtrioventricular Pathway Atrial Insertion Sites,” Circulation, Vol. 87,no. 2, pgs. 487-499 (February 1993).

Ablation of a specific location within or near the heart requires theprecise placement of the ablation catheter. Precise positioning of theablation catheter is especially difficult because of the physiology ofthe heart, particularly because the heart continues to beat throughoutthe ablation procedures. Commonly, the choice of placement of thecatheter is determined by a combination of electrophysiological guidanceand fluoroscopy (placement of the catheter in relation to known featuresof the heart, which are marked by radiopaque diagnostic catheters thatare placed in or at known anatomical structures, such as the coronarysinus, high right atrium, and the right ventricle).

Ablation procedures using guiding introducers to guide an ablationcatheter to a particular location in the heart for treatment of atrialarrhythmia have been disclosed in, for example, U.S. Pat. Nos.5,427,119, 5,497,774, 5,564,440, 5,575,766, 5,628,316, and 5,640,955.During these procedures, ablation lesions are produced in the heart asan element of the medical procedure.

The energy necessary to ablate cardiac tissue and create a permanentlesion can be provided from a number of different sources. Originally,direct current was utilized although laser, microwave, ultrasound, andother forms of energy have also been utilized to perform ablationprocedures. Because of problems associated with the use of DC current,however, radiofrequency (RF) has become the preferred source of energyfor ablation procedures. The use of RF energy for ablation has beendisclosed, for example, in U.S. Pat. Nos. 4,945,912, 5,242,441,5,246,438, 5,281,213, 5,281,218, and 5,293,868. The use of RF energywith an ablation catheter contained within a transseptal sheath for thetreatment of W-P-W in the left atrium is disclosed in Swartz, J. F. etal., “Radiofrequency Endocardial Catheter Ablation of AccessoryAtrioventricular Pathway Atrial Insertion Sites,” Circulation, Vol. 87,pgs. 487-499 (1993). See also Tracey, C. N., “Radio Frequency CatheterAblation of Ectopic Atrial Tachycardia Using Paced Activation SequenceMapping,” J. Am. Coll. Cardiol. Vol. 21, pgs. 910-917 (1993).

In addition to radiofrequency ablation catheters, thermal ablationcatheters have been disclosed. During thermal ablation procedures, aheating element, secured to the distal end of a catheter, heatsthermally conductive fluid, which fluid then contacts the human tissueto raise its temperature for a sufficient period of time to ablate thetissue. A method and device for thermal ablation using heat transfer isdisclosed in U.S. Pat. No. 5,433,708. Another thermal ablation procedureutilizing a thermal electrode secured to a catheter and located within aballoon with openings in that balloon to permit heated conductive fluidintroduced into the balloon from the catheter to escape from the balloonfor contact with the tissue to be ablated is disclosed in U.S. Pat. No.5,505,730.

Conventional ablation procedures utilize a single distal electrodesecured to the tip of an ablation catheter. Increasingly, however,cardiac ablation procedures utilize multiple electrodes affixed to thecatheter body. These ablation catheters often contain a distal tipelectrode and a plurality of ring electrodes as disclosed in, forexample, U.S. Pat. Nos. 4,892,102, 5,228,442, 5,327,905, 5,354,297,5,487,385, and 5,582,609.

To form linear lesions within the heart using a conventional ablationtip electrode requires the utilization of procedures such as a “dragburn.” The term “linear lesion” as used herein means and elongate,continuous lesion, whether straight or curved, that blocks electricalconduction. During a “drag burn” procedure, while ablating energy issupplied to the tip electrode, the tip electrode is drawn across thetissue to be ablated, producing a line of ablation. Alternatively, aseries of points of ablation are formed in a line created by moving thetip electrode incremental distances across the cardiac tissue. Theeffectiveness of these procedures depends on a number of variablesincluding the position and contact pressure of the tip electrode of theablation catheter against the cardiac tissue, the time that the tipelectrode of the ablation catheter is placed against the tissue, theamount of coagulum that is generated as a result of heat generatedduring the ablation procedure, and other variables associated with abeating heart, especially an erratically beating heart. Unless anuninterrupted track of cardiac tissue is ablated, unablated tissue orincompletely ablated tissue may remain electrically active, permittingthe continuation of the stray circuit that causes the arrhythmia.

It has been discovered that more efficient ablation may be achieved if alinear lesion of cardiac tissue is formed during a single ablationprocedure. The production of linear lesions in the heart by use of anablation catheter is disclosed in, for example, U.S. Pat. Nos.5,487,385, 5,582,609, and 5,676,662. A specific series of linear lesionsformed in the atria for the treatment of atrial arrhythmia are disclosedin U.S. Pat. No. 5,575,766.

The ablation catheters commonly used to perform these ablationprocedures produce electrically inactive or noncontractile tissue at aselected location by physical contact of the cardiac tissue with anelectrode of the ablation catheter. Conventional tip electrodes withadjacent ring electrodes cannot perform this type of procedure, however,because of the high amount of energy that is necessary to ablatesufficient tissue to produce a complete linear lesion. Also,conventional ring electrode ablation may leave holes or gaps in alesion, which can provide a pathway along which unwanted circuits cantravel.

An ablation catheter for use in the heart that contains a pair ofintertwined helical electrodes is disclosed in U.S. Pat. No. 5,334,193.The helically wound electrode is affixed to the surface of the catheterbody over a distance of about eight centimeters from the distal tip ofthe catheter body. Other helical electrodes are disclosed in U.S. Pat.Nos. 4,161,952, 4,776,334, 4,860,769, 4,934,049, 5,047,026, 5,542,928,and WO 95/10319.

During conventional ablation procedures, the ablating energy isdelivered directly to the cardiac tissue by an electrode on the catheterplaced against the surface of the tissue to raise the temperature of thetissue to be ablated. This rise in tissue temperature also causes a risein the temperature of blood surrounding the electrode, which oftenresults in the formation of coagulum on the electrode, which reduces theefficiency of the ablation electrode. With direct contact between theelectrode and the blood, some of the energy targeted for the tissueablation is dissipated into the blood.

To achieve efficient and effective ablation, coagulation of blood thatis common with conventional ablation catheters should be avoided. Thiscoagulation problem can be especially significant when linear ablationlesions or tracks are produced because such linear ablation proceduresconventionally take more time than ablation procedures ablating only asingle location.

In some instances, stray electrical signals find a pathway down thepulmonary veins and into the left atrium of the heart. In theseinstances, it may be advantageous to produce a circumferential lesion ator near the ostium of one or more of the pulmonary veins. Desirably,such a circumferential lesion would electrically isolate a pulmonaryvein from the left atrium, completely blocking stray signals fromtraveling down the pulmonary vein and into the left atrium. It isdesirable to have a catheter with a distal portion for forming suchcircumferential lesions in tissue while avoiding problems with existingdesigns.

BRIEF SUMMARY OF THE INVENTION

It is an object of the disclosed invention to provide an improvedablation catheter for forming linear lesions in tissue, including tissuewithin the human heart and at or near the ostium of the pulmonary veins.This and other objects are provided by the ablation catheter that isdisclosed by the present invention.

In a first form, the instant invention is an ablation catheter fortreating tissue. The ablation catheter comprises a porous conductoradapted to deliver therapeutic energy (e.g., the porous conductor may beconnected to an RF generator) and a catheter shaft. The catheter shafthas a proximal portion and a distal portion. The distal portioncomprises at least one lumen (which is adapted to carry wires, opticalfibers, and fluids for a variety of functional purposes) and an activeregion. The porous conductor is attached within the at least one lumen,and the active region is adapted to introduce the therapeutic energyfrom the porous conductor to the tissue. The catheter shaft may beconstructed from a polymer, and the porous conductor may be constructedfrom metal.

In another form, the instant invention is a catheter for ablating tissueand, in one form, comprises a catheter shaft and a mesh or wovenelectrode. The mesh or woven electrode is adapted to be connected to anablation energy supply (e.g., an RF generator). The catheter shaft has aproximal portion, a distal portion, and at least a first lumen thatextends from the proximal portion to the distal portion. The distalportion is adapted to be inserted into a body cavity having tissue to beablated and may be straight or curved. The distal portion comprises anactive region that includes at least one porthole. The first lumen isadapted to carry a conductive fluid medium from the proximal portion tothe portholes along the active region of the distal portion. The mesh orwoven electrode is mounted within the first lumen over the plurality ofportholes and extends along the active region of the distal portion, andthe mesh or woven electrode is adapted to supply the ablation energy tothe tissue through the conductive fluid medium.

In yet another form, the instant invention is again a catheter forablating tissue and comprises a catheter shaft and a mesh electrode. Thecatheter shaft comprises a proximal portion, a distal portion, a firstlumen, and a second lumen. The distal portion comprises at least onecurved section adapted to be inserted into a body cavity having tissueto be ablated. The at least one curved section defines an innerperipheral wall and an outer peripheral wall, and the outer peripheralwall has an active region that includes a plurality of portholes. Thefirst lumen extends from the proximal portion to the distal portion, andthe first lumen is adapted to carry a conductive fluid medium from theproximal portion to the portholes along the active region of the distalportion. The second lumen extends adjacent to the inner peripheral wall.The mesh electrode is mounted within the first lumen over the pluralityof portholes and extends along the active region of the distal portion.The mesh electrode is adapted to supply ablation energy through theconductive fluid medium. A shape retention wire is mounted in the secondlumen.

In another form of the instant invention, the cross-sectionalconfiguration of the active region of the distal portion is adapted tobias the active region against the tissue to be ablated. Thus, thecross-sectional configuration along the active region may comprise aflattened outer peripheral wall. Such cross-sectional configurationsinclude polygonal configurations. As used herein, a “polygonalconfiguration” may include a curved line segment or a curved side. Thus,D-shaped, triangular, or rectangular cross-sectional configurations areall polygonal configurations as that term is used herein.Cross-sectional configuration having a flattened outer peripheral wallmay also include, for example, elliptical configurations.

A more detailed explanation of the invention is provided in thefollowing description and claims, and is illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an ablation catheter assembly includingan ablation catheter according to a first embodiment of the presentinvention.

FIG. 2 is a fragmentary view of a distal portion of the ablationcatheter according to the first embodiment of the present invention,wherein the active region of the catheter is curved.

FIG. 3 is a fragmentary view of an ablation catheter according to afirst variant of the first embodiment of the present invention, takenalong line 3-3 of FIG. 2, wherein pieces of the ablation catheter wallhave been broken away to reveal internal features of a bi-lumenal distalportion.

FIG. 4 is an enlarged, fragmentary, isometric view taken along line 4-4of FIG. 3 with pieces of the ablation catheter wall broken away toreveal the configuration of the bi-lumenal distal portion, with asection of porous conductor in a first lumen over a plurality ofportholes, and a shape retention wire in a second lumen.

FIG. 5 is similar to FIG. 4, but depicts an ablation catheter accordingto a second variant of the first embodiment of the present invention,with a distal portion having a single lumen carrying the porousconductor.

FIG. 6 is a fragmentary view of a distal portion of an ablation catheteraccording to a second embodiment of the present invention, wherein theactive region of the ablation catheter is straight.

FIG. 7 is an enlarged, fragmentary view of a portion of the ablationcatheter depicted in FIG. 6.

FIG. 8 is a fragmentary, isometric view of the distal portion of theablation catheter depicted in FIGS. 6 and 7, with a portion of an innerperipheral wall broken away to reveal a porous conductor positioned overthe plurality of portholes.

FIG. 9 is an isometric view of a heart with portions of the atria andventricles broken away to reveal positioning of the ablation catheterdepicted in, for example, FIGS. 1-4 (or the ablation catheter depictedin FIG. 5) in the left atrium, adjacent to the left superior pulmonaryvein.

FIG. 10 is similar to FIG. 9, but depicts the ablation catheter inposition near the ostium of the left superior pulmonary vein.

FIG. 11 is a fragmentary, isometric view similar to FIG. 5, but whereinthe active region of the ablation catheter is in position against tissueto be ablated, and wherein a conductive fluid medium is present in thefirst lumen, and wherein RF energy is being supplied to the tissue bythe porous conductor.

FIG. 12 is similar to FIG. 11, but depicts a section of the activeregion of the distal portion of an ablation catheter according to athird variant of the first embodiment of the present invention, whereinthe active region has a circular cross section with a constant wallthickness.

FIG. 13 is similar to FIG. 12, but depicts a section of the activeregion of the distal portion of an ablation catheter according to afourth variant of the first embodiment of the present invention, whereinthe active region has a D-shaped cross section with a constant wallthickness.

FIG. 14 is similar to the embodiment depicted in FIG. 13, but depicts asection of a distal portion of an ablation catheter according to a fifthvariant of the first embodiment of the present invention, wherein thedistal portion has a piece of the inner peripheral wall broken away toreveal a porous conductor in position over the portholes.

FIG. 15 is similar to FIG. 13, but depicts a section of a distal portionof an ablation catheter according to a sixth variant of the firstembodiment of the present invention, wherein the active region of thedistal portion has a triangular cross section.

FIGS. 16-19 depict various fragmentary, cross-sectional views ofportions of a pulmonary vein and portions of the left atrium, with asection of the ablation catheter embodiment depicted in FIG. 13 or FIG.14 in place against the ostium or the inner wall of the pulmonary vein.

DETAILED DESCRIPTION OF THE INVENTION

In general, the instant invention relates to an ablation catheter 10,which may comprise part of an ablation catheter assembly 12, wherein theablation catheter 10 comprises a catheter shaft 14 having a proximalportion 16 and a unique distal portion 18 (see, e.g., FIGS. 1-3) or 18′(see, e.g., FIGS. 6-8) for ablating tissue 20 (see, e.g., FIG. 11) usingenergy 22 emanating from a porous conductor (e.g., mesh or woven) 24(see, e.g., FIGS. 3-5) or 24′ (see, e.g., FIG. 8) attached within theablation catheter 10, and/or wherein the distal portion of the ablationcatheter 10 may have a cross-sectional configuration that is adapted tobias the catheter into a desired orientation which places a flattenedouter peripheral wall 26, 26′ (see, e.g., FIGS. 13-15) of an activeregion 38 (see, e.g., FIGS. 2 and 3) or 38′ (see, e.g., FIG. 6) of thecatheter against the tissue 20 to be ablated. As used herein,“flattened” outer peripheral walls encompasses more than merely “flat”outer peripheral walls. For example, some oval or ellipticalconfigurations have at least one flattened wall within the meaning ofthat term as used herein. The catheter shaft 14 may be constructed froma number of different polymers (e.g., polyurethane, polypropylene,oriented polypropylene, polyethylene, crystallized polyethyleneterephthalate, polyethylene terephthalate, polyester, polyvinylchloride, etc.).

FIG. 1 is an isometric view looking downwardly at an ablation catheterassembly 12 having an ablation catheter 10 according to a firstembodiment of the present invention. In the first embodiment, the distalportion 18 of the ablation catheter 10 is curved (see also, e.g., FIGS.2-5). As depicted in FIG. 1, the ablation catheter 10 may be used incombination with an inner guiding introducer 28 and an outer guidingintroducer 30. Alternatively, a single guiding introducer may be used ora precurved transseptal sheath may be used instead of one or moreguiding introducers. In general, the guiding introducer, the guidingintroducers, or the precurved sheath are shaped to facilitate placementof the ablation catheter 10 at the tissue 20 to be ablated. Thus, forexample, the introducer or the introducers or the transseptal sheathmake it possible to navigate to the heart 32 and through its complexphysiology to reach specific tissue to be ablated. FIGS. 6-8 depict asecond embodiment of an ablation catheter 10 according to the presentinvention.

As shown in FIGS. 1-8, the ablation catheter 10 according to the presentinvention may have a curved distal portion 18 (see, e.g., FIGS. 1-3) ora straight distal portion 18′ (see, e.g., FIGS. 6-8). The distalportion, whether curved or straight, includes one or more lumens 34, 36to carry wires, optical fibers, or fluids (e.g., a conductive fluid or aradiopaque fluid) for a variety of functional purposes, and an activeregion 38 (see, e.g., FIGS. 2 and 3) or 38′ (see, e.g., FIG. 6) thatperforms the actual ablation of tissue. The wires that may be present inthe lumens may include, for example, metallic or nonmetallic wires thatprovide support or that enhance the positionability of the distalportion (e.g., shape retention wires 40 (see, e.g., FIGS. 3 and 4) orshape memory wires or super elastic wires). The wires may also be usedfor conducting diagnostic electrical signals from the distal portion ortherapeutic energy to the distal portion. In both of the embodiments, aplurality of portholes 44-48 (FIGS. 2-5 and FIGS. 6-8) extend along aporthole centerline 42 (see, e.g., FIG. 2) or 42′ (see, e.g., FIGS. 6and 7) in the active region 38 (see, e.g., FIG. 2) or 38′ (see, e.g.,FIG. 6). The portholes include a most proximal or first porthole 44(see, e.g., FIG. 3) or 44′ (see, e.g., FIG. 6), a most distal or lastporthole 46 (see, e.g., FIGS. 2 and 3) or 46′ (see, e.g., FIGS. 6 and7), and a plurality of intermediate portholes 48 (see, e.g., FIGS. 2-5)or 48′ (see, e.g., FIGS. 6-8). The porthole centerline 42, 42′ extendsalong an outer peripheral wall 50 (see, e.g., FIGS. 3-5) or 50′ (see,e.g., FIGS. 6 and 7) of the distal portion, parallel to the longitudinalaxis 52 (see, e.g., FIG. 12), 52′ (see, e.g., FIGS. 13 and 14), or 52″(see, e.g., FIG. 15) of the portion of the ablation catheter definingthe active region.

As shown in FIGS. 1-5, in the first embodiment of the ablation catheter10, the distal portion 18 comprises a first curved section 54, a secondcurved section 56, and a third curved section 58, which togethercomprises a unitary component in this embodiment, but which couldcomprise separate pieces that have been joined together. A rounded tip60, which may be an ablation electrode, is clearly visible in FIGS. 2and 3. The catheter shaft 14, which is typically a braided shaft,includes a “straight” section 62 (see, e.g., FIG. 2) that follows acircuitous path from the location of the distal portion 18 of thecatheter shaft 14, which is adjacent to the tissue to be ablated, to theproximal portion 16 of the catheter shaft 14, which is outside of thebody containing the tissue to be ablated. The straight section 62 isjoined to the distal portion 18. In this first embodiment, the thirdcurved section 58 comprises the active region 38. As shown to goodadvantage in FIGS. 2 and 3, in the first embodiment the active region 38is along a radial apex of the outer peripheral wall 50, along theporthole centerline 42. The active region 38 of the distal portion 18 isthe portion that includes the plurality of portholes 44-48 that areplaced against the tissue 20 to be ablated (e.g., the inner wall of apulmonary vein).

FIGS. 3 and 4 depict a first variant of the first embodiment of theablation catheter 10 depicted in FIGS. 1 and 2. In this first variant ofthe first embodiment, the ablation catheter 10 is a virtual electrodeablation catheter having a bi-lumenal distal portion 18, including afirst lumen 34 adjacent to the outer peripheral wall 50 and a secondlumen 36 adjacent to an inner peripheral wall 64. FIG. 3 is afragmentary view of the distal portion 18 of the ablation catheter takenalong line 3-3 of FIG. 2, wherein pieces of the ablation catheter wallhave been broken away to reveal internal features of the bi-lumenaldistal portion 18. FIG. 4 is an enlarged, fragmentary, isometric viewtaken along line 4-4 of FIG. 3 with pieces of the ablation catheter wallbroken away. As clearly shown in FIGS. 3 and 4, the first variant of thefirst embodiment includes a porous conductor 24 (e.g., a metal mesh orwoven electrode) mounted on the inside of the first lumen 34 over theplurality of portholes 44-48, thereby forming a porous fluiddistribution manifold. The second lumen 36 in the embodiment of FIGS. 3and 4 includes a shape retention wire 40 (e.g., a Nitinol or NiTi wire).The first lumen 34 is adapted to carry a conductive fluid medium 66(e.g., hypertonic saline) during use of the ablation catheter. Theconductive fluid medium may be seen in, for example, FIG. 11. Anelectrical lead 68 supplies ablation energy 22 to the porous conductor24. This electric lead 68 has one end connected to the porous conductor24 at the distal portion 18 of the ablation catheter 10, and itsopposite end connected to an energy source (not shown) in a knownmanner, at the proximal portion 16 of the ablation catheter assembly 12depicted in FIG. 1.

FIG. 5 depicts a second variant of the first embodiment of an ablationcatheter according to the present invention. In this second variant, thedistal portion of the ablation catheter has only a first lumen 34. Inthis particular variant of the first embodiment, the distal portion ofthe ablation catheter may be either manufactured from materials thatsufficiently retain a desired configuration, possibly attributable toone or more thickened areas 70, or it is unnecessary for the distalportion of the ablation catheter to hold a specific configuration.

FIGS. 6-8 depict a fragmentary view of the distal portion 18′ of theablation catheter 10, according to the second embodiment of the presentinvention, wherein the ablation catheter 10 again is a virtual electrodeablation catheter. The active region 38′ of the ablation catheteraccording to the second embodiment is straight. In FIG. 8, which is afragmentary, isometric view of the second embodiment of the distalportion of the ablation catheter according to the present invention, apiece of the inner peripheral wall 64′ has been broken away to reveal aporous conductor 24′ in position over the portholes 48′. In this secondembodiment of the distal portion 18′ of the ablation catheter 10according to the present invention, the ablation catheter 10 has atleast one lumen in which conductive fluid medium can flow from theproximal portion of the ablation catheter to the distal portion of theablation catheter. The conductive fluid medium would flow through theporous conductor 24′ and exit the distal portion 18′ of the ablationcatheter 10 through the plurality of portholes 44′-48′ as discussedfurther below. A rounded tip 60, which may be an ablation electrode, mayalso be seen in FIGS. 6-8.

The porous conductor 24 (see, e.g., FIGS. 3-5) or 24′ (see, e.g., FIG.8) may be mounted (e.g., bonded or frictionally fit) in the ablationcatheter 10 after it is formed, or the ablation catheter 10 may beformed around the porous conductor. If the porous conductor is mountedin a formed ablation catheter, a tapered mandrel may be used to placethe porous conductor into, and conform it to, the interior configurationof the appropriate lumen. The portholes may be formed (e.g., molded ordrilled) before or after the porous conductor is mounted. The porousconductor may overlay the entire inner surface or less than the entireinner surface of the lumen in which the porous conductor is mounted.

Remaining FIGS. 9-19 depict the ablation catheter 10 according to thepresent invention in use, for example, ablating tissue in a pulmonaryvein. FIGS. 9 and 10 depict a number of primary components of the heart32 to orient the reader. In particular, starting in the upper left handportion of FIGS. 9 and 10, and working around the periphery of the heartin a counterclockwise direction, the following parts of the heart aredepicted: superior vena cava 72, right atrium 74, inferior vena cava 76,right ventricle 78, left ventricle 80, left inferior pulmonary vein 82,left superior pulmonary vein 84, left atrium 86, right superiorpulmonary vein 88, right inferior pulmonary vein 90, left pulmonaryartery 92, arch of aorta 94, and right pulmonary artery 96.

The distal portion 18 of the ablation catheter 10 according to the firstembodiment, for example, is positioned adjacent to the ostium 98 of theleft superior pulmonary vein 84 (see FIG. 9) using known procedures,like the “Seldinger technique,” wherein the right venous system may befirst accessed as follows. A peripheral vein (such as a femoral vein) ispunctured with a needle, the puncture wound is dilated with a dilator toa size sufficient to accommodate a guiding introducer or transseptalsheath. The guiding introducer or transseptal sheath with at least onehemostasis valve (see FIG. 1) is seated within the dilated puncturewound while maintaining relative hemostasis. With the guiding introduceror transseptal sheath in place, the ablation catheter 10 is introducedthrough the hemostasis valve of the guiding introducer or transseptalsheath and is advanced along the peripheral vein, into the region of thevena cava (e.g., the inferior vena cava 76), and into the right atrium74. From there, the ablation catheter 10, together with its guidingintroducer or transseptal sheath is further advanced through a hole inthe interatrial septum, which a doctor would make before inserting theablation catheter 10 into the guiding introducer or transseptal sheath,and into the left atrium 86. Once the guiding introducer or transseptalsheath is in the left atrium 86, it can be advanced to the respectivepositions depicted in FIGS. 9 and 10. The ablation catheter 10 caneither be advanced until the active region 38 of the distal portion 18extends from the guiding introducer or the transseptal sheath, or theguiding introducer or the transseptal sheath can be retracted to exposethe distal portion 18 of the ablation catheter 10. In FIG. 10, thedistal portion 18 of the ablation catheter 10 according to the firstembodiment is near the ostium 98 of the left superior pulmonary vein 84.

While the distal portion 18 of the ablation catheter 10 is near theostium 98 of the left superior pulmonary vein 84 as depicted in FIG. 10,the porous conductor 24 (see, e.g., FIGS. 3-5) is activated to create adesired lesion. As shown in FIG. 11, during activation of the ablationcatheter, a conductive fluid medium 66 is flowing through the firstlumen 34, past the porous conductor 24, and out of the portholes 44-48.The porous conductor 24, when the ablation catheter is active, deliversablation energy 22 (e.g., radiofrequency or RF energy) to the tissue 20via the conductive fluid medium 66. The RF energy 22 emanating from theporous conductor 24 passes through the conductive fluid medium 66contained in the first lumen 34, through the portholes 44-48, and intothe adjacent tissue 20. Thus, when the ablation catheter 10 is operatingwith conductive fluid medium 66 flowing through the porous conductor 24and out of the portholes 44-48, the ablation energy 22 is delivereddirectly to the tissue 20 through the portholes 44-48. In thisembodiment, a lesion is formed in the tissue 20 by the RF energy 22.Lesion formation may also be facilitated or enhanced by the conductivefluid medium 66, which convectively cools the surface of the tissue 20while the ablation energy 22 is being delivered below the surface of thetissue. This inhibits excess damage to the surface of the tissue 20while also reducing the amount of coagulum formed. The RF energy 22 isconducted into the adjacent tissue 20 while the conductive fluid medium66 convectively cools the surface of the tissue 20.

In order for the ablation catheter to form a sufficient lesion, it isdesirable to raise the temperature of the tissue to at least 50-60° C.for an appropriate length of time. Thus, sufficient RF energy must besupplied to the porous conductor to produce this lesion-formingtemperature in the adjacent tissue for the desired duration. When theflow rate of the conductive fluid medium is appropriately regulated, theconductive fluid medium flows at a sufficient rate to avoid stagnationor re-circulation and to push blood away from the gap between thecatheter and the tissue. The flow rate should be high enough to preventor minimize vaporization of the conductive fluid medium since suchvaporization can inhibit delivery of ablation energy to the tissue. Aspreviously mentioned, the distal portion of the ablation catheter formsthe lesion by direct conduction of ablation energy from the porousconductor through the conductive fluid medium and into the tissue.

The conductive fluid medium flowing through the porous conductor andportholes prevents blood from flowing into the distal portion of theablation catheter and pushes blood from the area adjacent to theportholes. This helps prevent formation of coagulum, which can haveundesirable effects on the patient. As mentioned above, the conductivefluid medium is caused to flow at a rate that prevents the electrodefrom overheating the conductive fluid medium and producing vapor in thefirst lumen. If the conductive fluid medium were to boil, creatingvapor, the ablation catheter's ability to form a desired lesion in theadjacent tissue may be greatly reduced or inhibited since the ablationenergy may be unable to reach the tissue in sufficient quantity. Thus,the flow of conductive fluid medium through the first lumen, the porousconductor, and out of the portholes is managed or regulated so thatthere is sufficient flow to prevent vaporization, but not so much flowthat the gap between the catheter and the tissue opens, prohibiting theporous conductor from being able to deliver sufficient energy to theadjacent tissue to form a desired lesion. If the gap between thecatheter and the tissue becomes too great, an undesirable amount of theablation energy may pass to the blood rather than to the tissue. Also,if the conductive fluid medium flows out the portholes at too high of aflow rate, the composition of the patient's blood may be adverselyeffected by the excess quantity of conductive fluid medium being mixedwith the patient's blood.

The desired flow rate of the conductive fluid medium is achieved byadjusting, for example, the pressure pushing the conductive fluid mediumthrough the first lumen, changing the size of the first lumen, changingthe finish on the inner wall of the first lumen, changing the size ordistribution of the portholes, changing the cross-sectionalconfiguration of the portholes, altering the spacing 100 (FIG. 7)between the portholes, and/or changing the porthole diameter gradientbetween the first porthole and the last porthole whenever such agradient exists. Another factor that may be taken into account whenadjusting the flow rate of the conductive fluid medium is theconfiguration of the porous conductor. For example, the size of the gapsor pores may be adjusted when trying to establish a satisfactory flowrate through the distal portion of the ablation catheter. The porousconductor may significantly restrict the flow of the conductive fluidmedium from the portholes. A metal mesh electrode with a mesh gap sizeof about 10-50 micrometers may permit a desired flow rate of theconductive fluid medium, for example. The specific configuration of thedistal portion of the ablation catheter can also influence the flow rateof the conductive fluid medium. For example, in the first embodiment ofthe ablation catheter (see, e.g. FIGS. 1-3), the radius of curvature ofthe active region 38 of the distal portion 18 affects the tendency ofthe conductive fluid medium 66 to flow out of the portholes 44-48.

FIG. 12 is a fragmentary, isometric view of a portion of the activeregion of an ablation catheter 10 according to a third variant of thefirst embodiment of the present invention. In this variant, the ablationcatheter 10 has a circular cross section and walls of a constantthickness (compare FIG. 5 wherein the catheter wall has a thickened area70), but the walls could be of a changing or variable thickness. This isa traditional, axisymmetric round extrusion. In FIG. 12, the ablationcatheter is positioned to create a desired lesion, with the activeregion of the ablation catheter extending around or encircling thelongitudinal axis 102 of a pulmonary vein, for example. With theablation catheter in this position, the longitudinal axis 52 of theactive region of the ablation catheter encircles the longitudinal axis102 of the pulmonary vein. Since the internal anatomy of veins variesgreatly, and since it is difficult to align the active region of theablation catheter such that the longitudinal axis 102 of the pulmonaryvein is precisely aligned with the longitudinal axis 104 of the cathetershaft 14, it is possible that the portholes 44-48 will not rest asdirectly against the internal surface 20 of the pulmonary vein as may bedesired.

As shown in FIG. 12, this makes it possible for the active region toroll or move when placed on an irregular surface, which permits theconductive fluid medium and RF energy to asymmetrically exit theportholes as indicated by the arrows 106, 108 on FIG. 12 and to exit theportholes more easily than desired. This in turn can lead to lesseffective operation of the ablation catheter 10. In other words, whenthe portholes 44-48 through which the conductive fluid medium 66 exitsthe ablation catheter are pressed precisely and solidly against theinternal surface of the pulmonary vein (e.g., the left superiorpulmonary vein 84 shown in FIG. 12), a better lesion may be formed. Onthe other hand, when the outer peripheral wall 50 (see, e.g., FIGS. 3-5)of the ablation catheter rests on the internal surface of the pulmonaryvein at an angle, as shown in FIG. 12, an opportunity is presented forthe conductive fluid medium and RF energy to asymmetrically and easilyescape from the region between the ablation catheter 10 and the tissue20 comprising the inner wall of the pulmonary vein 84, producing a lowerquality lesion. Thus, it is desirable to configure the active region ofthe distal portion of the ablation catheter such that the outerperipheral wall of the active region is biased against the tissue to beablated.

FIGS. 13-19 depict cross-sectional configurations that are notcompletely axisymmetric about the longitudinal axis of the active regionof the ablation catheter. These cross-sectional configurations arebiased toward a preferred orientation that places the outer peripheralwall, and thus the active region of the catheter (e.g., the portholes,if present), squarely against the tissue to be ablated. When the outerperipheral wall is biased against the ostium or the inner wall of thepulmonary vein, the active region of the ablation catheter is easier toposition and more stable during operation. If one or more portholes arepresent and conductive fluid medium is flowing through the portholes,manifolding of the conductive fluid medium is improved, and blood may bemore effectively isolated from the tissue to be ablated.

In the fourth and fifth variants of the first embodiment depicted inFIGS. 13 and 14, respectively, the active region of the ablationcatheter has a D-shaped cross-section. As shown in FIG. 13, whichdepicts the fourth variant of the first embodiment (no porous conductorpresent), when the ablation catheter having this cross-sectionalconfiguration first contacts the tissue to be ablated (phantom lines inFIG. 13), it is biased in the direction of the two curved arrows 110,112 depicted in FIG. 13 to torque and rotate the entire outer peripheralwall 26 into direct contact with the tissue 20 to be ablated (solidlines in FIGS. 13 and 14). This cross-sectional configuration for theactive region of the distal portion of the ablation catheter thus helpsorient the outer peripheral wall 26 against the tissue 20 to be treatedor diagnosed. In the depicted embodiment, portholes 44-48 pass throughthis outer peripheral wall 26. Thus, when the outer peripheral wall 26is biased against the tissue 20 to be ablated, the portholes are bestoriented to achieve the desired lesion. In FIG. 14, which depicts thefifth variant of the first embodiment, the D-shaped cross-sectionalconfiguration is shown again, but a piece of the inner peripheral wall64 has been broken out to reveal a porous conductor 24 in position inthe lumen of the ablation catheter over the portholes. For theconfiguration depicted in FIGS. 13 and 14, an aspect ratio of at least1.5:1 and preferably of 2.2:1 has been found to work well.

FIG. 15 depicts a sixth variant of the first embodiment, which isanother possible cross-sectional configuration for the active region ofthe distal portion of a catheter that would bias the outer peripheralwall 26′ of the catheter against the tissue 20 to be diagnosed ortreated (e.g., ablated). As shown in FIG. 15, when the active regionfirst contacts the tissue 20 (phantom lines in FIG. 15), the outerperipheral wall 26′ may not be as fully seated as possible against thetissue 20. The triangular cross-sectional configuration depicted in FIG.15, however, again biases the outer peripheral wall 26′ in the directionof the two curved arrows 110′, 112′ depicted in FIG. 15, to torque androtate the entire outer peripheral wall 26′ against the tissue 20,driving the distal portion 18 of the catheter 10 toward the orientationdepicted in solid lines in FIG. 15. Other configurations that bias theouter peripheral wall against the tissue are possible and include anyconfigurations that have a substantially flattened outer peripheralwall. For example, a rectangular cross-sectional configuration and othermulti-side cross-sectional configurations that includes at least oneflattened outer peripheral wall would also bias the outer peripheralwall against the tissue 20. As previously alluded to, the orientationbiasing configurations (e.g., those depicted in FIGS. 13-15) may be usedin devices with or without a fluid distribution manifold like the seriesof portholes 44-48 depicted in these figures.

FIGS. 16-19 depict an ablation catheter having a cross-sectionalconfiguration shown to best advantage in FIGS. 13 and 14 being usedduring pulmonary vein ablation. As shown in FIGS. 16-19, the ostium 98of a pulmonary vein 84 may have a variety of irregular shapes. In FIG.16, the side walls of the pulmonary vein 84 are substantially parallel,but the walls do not remain parallel adjacent to the ostium 98, wherethe pulmonary vein connects to the left atrium 86. As shown in FIG. 16,a catheter having a third curved section 58 with a biasingcross-sectional configuration (e.g., the D-shaped configuration depictedin this figure and FIGS. 13 and 14) is able to twist about thelongitudinal axis 52′ of the active region and about the longitudinalaxis 104 of the catheter shaft 14, thereby better ensuring that theouter peripheral wall 26 of the distal portion of the ablation catheteris seated against the tissue 20 to be ablated. FIG. 17 depicts yetanother possible anatomy for the pulmonary vein 84, ostium 98, and leftatrium 86. In this figure, the side walls of the pulmonary vein divergenear the ostium. The active region of the distal portion is again biasedagainst the inner wall of the pulmonary vein near the ostium 98 by thecross-sectional configuration of the active region. FIGS. 18 and 19provide additional views of possible variations in the anatomy of ahuman heart, and, again, the active region of the distal portion of theablation catheter is biased against the tissue 20 to be ablated.Clearly, there are as many possible heart anatomies as there are hearts.

The unique cross-sectional configurations depicted in FIGS. 13-19,having a flattened outer peripheral wall 26, 26′ that enhances contactbetween the ablation catheter and the tissue to be ablated, are notlimited to use with ablation catheters employing virtual electrodes likethe porous conductor 24. The cross-sectional configurations depicted inFIGS. 13-19, could be used with ablation catheters that generateablation energy by other than a porous conductor (e.g., a flat wireelectrode and a coiled wire electrode). These biasing configurationsenhance the performance of catheters having diagnostic or therapeuticelectrodes, including actual electrodes (e.g., traditional ringelectrodes), or virtual electrodes, or other energy sources that need tobe accurately oriented relative to selected tissue.

Although FIGS. 13-19 are described in connection with an ablationcatheter assembly, the unique cross-sectional configurations shown inthese figures and discussed above for biasing an active region of acatheter against tissue could also be used with diagnostic catheters orother catheters that do not ablate tissue.

Although preferred embodiments of this invention have been describedabove with a certain degree of particularity, those skilled in the artcould make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this invention. For example, theporous conductor described above could be used with another electricalelement. In such an embodiment, the mesh or weave may not distributeenergy, thereby comprising, for example, a passive part of a fluiddistribution manifold. In contrast, the porous conductor described aboveboth comprises part of the fluid distribution manifold and distributesenergy. Also, the drawings disclose a distal portion of the catheterthat includes a plurality of circular portholes, but the portholes neednot be circular, and a single, elongated porthole may be used in placeof the depicted plurality of portholes. All directional references(e.g., upper, lower, upward, downward, left, right, leftward, rightward,top, bottom, above, below, vertical, horizontal, clockwise, andcounterclockwise) are only used for identification purposes to aid thereader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. It is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative only and not limiting. Changes in detail or structuremay be made without departing from the spirit of the invention asdefined in the appended claims.

1. A method of treating tissue using a virtual electrode ablationcatheter, the method comprising dispensing a conductive fluid mediumthrough a porous conductor within the virtual electrode ablationcatheter and onto tissue adjacent to portholes formed through a distalportion of the virtual electrode ablation catheter; activating theporous conductor to deliver ablation energy to the adjacent tissue viathe conductive fluid medium, the ablation energy emanating from theporous conductor and passing through the conductive fluid medium,through the portholes, and into the adjacent tissue; and inhibitingmovement of the distal portion of the virtual electrode ablationcatheter during delivery of the ablation energy.
 2. The method of claim1, wherein inhibiting movement further comprises biasing the distalportion of the virtual electrode ablation catheter toward a preferredorientation.
 3. The method of claim 1, wherein said dispensing stepfurther comprises symmetrically dispensing the conductive fluid mediumfrom the portholes.
 4. The method of claim 1, wherein the ablationenergy is delivered directly to the adjacent tissue through theportholes.
 5. The method of claim 1, wherein the ablation energy isdelivered below a surface of the adjacent tissue.
 6. The method of claim5, wherein the conductive fluid medium convectively cools the surface ofthe tissue while the ablation energy is being delivered below thesurface of the adjacent tissue.
 7. The method of claim 1, furthercomprising controlling a flow rate of the conductive fluid medium toavoid stagnation or re-circulation and to push blood away from betweenthe porous conductor and the adjacent tissue.
 8. The method of claim 7,wherein the flow rate is sufficiently high to reduce vaporization of theconductive fluid medium.
 9. The method of claim 7, wherein the flow rateis sufficiently high to push blood from an area adjacent to theportholes.
 10. The method of claim 7, wherein the flow rate issufficiently high to prevent an electrode from overheating theconductive fluid medium and producing vapor in a lumen carrying theconductive fluid medium.
 11. The method of claim 7, wherein the flowrate is regulated to prevent a gap between the porous conductor and theadjacent tissue from opening.
 12. The method of claim 7, whereincontrolling the flow rate is by at least one of the following: adjustingpressure pushing the conductive fluid medium through a lumen carryingthe conductive fluid medium, changing size of the lumen carrying theconductive fluid medium, changing finish on an inner wall of the lumencarrying the conductive fluid medium, changing size of the portholes,changing distribution of the portholes, changing cross-sectionalconfiguration of the portholes, changing spacing between the portholes,and changing porthole diameter gradient between a first of the portholesand a last of the portholes.
 13. The method of claim 7, whereincontrolling the flow rate is by changing a configuration of the distalportion of the virtual electrode ablation catheter.
 14. The method ofclaim 13, wherein changing a configuration of the distal portion of thevirtual electrode ablation catheter includes at least changing acurvature radius of the distal portion.
 15. A method of treating tissuewith a virtual electrode ablation catheter, the method comprisingdispensing a conductive fluid medium through portholes formed in thevirtual electrode ablation catheter onto an adjacent tissue; controllinga flow rate of the conductive fluid medium to avoid stagnation orre-circulation of the conductive fluid medium; and delivering ablationenergy via the conductive fluid medium, the ablation energy emanatingthrough the conductive fluid medium and through the portholes directlyonto the adjacent tissue.
 16. The method of claim 15, further comprisingbiasing a distal portion of the virtual electrode ablation cathetertoward a preferred orientation to reduce roll during delivery of theablation energy when placed on an irregular surface.
 17. The method ofclaim 15, further comprising dispensing the conductive fluid mediumsymmetrically from the portholes.
 18. The method of claim 15, whereinthe ablation energy is delivered below a surface of the adjacent tissue.19. The method of claim 15, wherein the conductive fluid mediumconvectively cools the surface of the tissue while the ablation energyis being delivered to the adjacent tissue.
 20. The method of claim 15,wherein controlling the flow rate of the conductive fluid medium pushesblood away from a gap between the virtual electrode ablation catheterand the adjacent tissue.
 21. A virtual electrode ablation cathetersystem for treating tissue, the system comprising means for dispensing aconductive fluid medium through portholes formed in a catheter means andonto an adjacent tissue; means for controlling flow of the conductivefluid medium to avoid stagnation or re-circulation of the conductivefluid medium in the catheter means; and means for delivering ablationenergy via the conductive fluid medium for emanating through theconductive fluid medium and directly onto the adjacent tissue.
 22. Thesystem of claim 21, further comprising means for pushing blood away fromthe adjacent tissue.